During drilling operation and hydrocarbon production/injection from/to subsurface oil and gas wells it is important to monitor the downhole conditions, e.g. to detect leaks, temperature changes and flow conditions, in the pipes casings and areas surrounding the well. This is, however, complicated by the fact that the wells usually includes a number of coaxial pipes separated by annular spaces containing pressurized fluids, produced hydrocarbons etc, and it is difficult to communicate signals e.g. both inside and outside a casing or production tubing while allowing the fluids to pass. A pressure leak from the annuli separating the tubing could saturate the ground formation, which is a potential dangerous situation. The saturation of the formation could make the well collapsing and loosing its barrier function.
One known solution for obtaining this is the use of penetrators combined with antennas or coils for communicating across the annuli between the pipes, as described in WO2009/040510, U.S. Pat. No. 7,165,618 and U.S. Pat. No. 6,684,952, but in environments potentially involving pressure differences between the annuli between the tubes and casings this introduces a risk of failure. Also the inductive coupler portions, corresponding to windings, discussed in in U.S. Pat. No. 7,165,618 and U.S. Pat. No. 6,684,952 are not themselves sufficiently protected against the environment.
U.S. Pat. No. 7,170,424,U.S. Pat. No. 5,008,664, U.S. Pat. No. 4,852,648 and U.S. Pat. No. 3,550,682 discuss the possibility of communicating through the pipe wall using induction coils, and U.S. Pat. No. 5,008,664 discuss the challenge of inherently inefficient coupling between two induction coils coaxially arranged, separated by a metal pressure barrier. There is, however, a trade off between the requirements for power transmission efficiency, signal transmission capacity and efficient sealing in the volatile high pressure downhole environment. To a certain degree the poor coupling efficiency in U.S. Pat. No. 5,008,664 is compensated for by use of low power electronic in the downhole equipment/gauge. Although a higher efficiency can be obtained with a non-magnetic metal section, as described in a corresponding solution in U.S. Pat. No. 7,063,148 the coil sizing is short and the casing thickness too thin, 5-10 mm, to be useable for high pressure situations. Another solution is described in US2009/0085701, and to some degree in U.S. Pat. No. 7,170,424 and U.S. Pat. No. 4,901,069, where a PEEK material or similar is provided between the antennas but the pipe wall serving as a pressure barrier is not positioned between the antennas. The theory of controlling magnetic fields through barriers is also discussed in U.S. Pat. No. 6,628,118.